Long Term Evolution (LTE) carrier aggregation (CA) is part of the LTE-Advanced standard, 3GPP Release 10. Carrier aggregation is a key feature of LTE-Advanced (LTE-A) that enables operators to create larger “virtual” carrier bandwidths for LTE services by combining separate spectrum allocations. The benefits of this aggregation include higher peak data rates and increased average data rates for users.
In inter-site CA data is delivered to UE on carriers of different sites, enhanced NodeBs (eNBs). The dividing of data to different eNBs takes place typically in the eNB controlling the Primary Cell (PCell), referred to herein as a Master enhanced NodeB (MeNB). A part of the data flow to the user equipment (UE) is transmitted via the MeNB using one or more carrier frequencies, while rest of the data flow is forwarded via an X2 interface to an eNB controlling a Secondary Cell (SCell), referred to herein by a slave eNB (SeNB). From the SeNB the data flow may be delivered to the UE using one or more other carrier frequencies.
Data-transport protocols employing ACK/NACKs and providing reliable in-sequence data delivery (e.g. TCP, RLC Acknowledged Mode, etc.) have restricted resources, e.g. numbering space, available for book-keeping of transmitted Protocol Data Units (PDUs). When these resources have been fully assigned to follow up transmitted PDUs, protocol stalling may occur and new data cannot be transmitted to the User Equipment (UE) until some of the book-keeping resources are freed.
The concept of inter-site CA introduces delay to the data by the data being transmitted over the X2 interface between the MeNB and SeNB. This may increase the delay to the PDUs delivered via the SeNB up to 20 ms. Additional delay may be caused by scheduling at the SeNB due to non-ideal flow control.
Due to the delays protocol stalling may be increased which again reduces data rates received by users.